A genetic approach is proposed in order to study the biological functions and the physiological regulation of the cyclic AMP-dependent protein kinase enzymes. Inbred strains of mice will be screened for various characteristics of the protein kinase system, in hopes of finding a variant which could be subjected to genetic analysis. The characteristics which will be screened for include: specific activities of both the protein kinase catalytic and cyclic AMP-binding activities, the heat stability of these two activities, the isozymes of both the protein kinase holoenzyme and its dissociated subunits that can be resolved by DEAE-cellulose chromatography, the Km's for the ATP substrate, and the developmental changes in the amount of phosphate incorporated into endogenous protein substrates.